1. Field of the Invention
The present invention relates to a multilayer board having perforations which are formed precisely in spite of a high aspect ratio of the perforations. The present invention also relates to a stacked circuit substrate to improve wiring density and having through-holes which are formed precisely in spite of a high aspect ratio of the through-holes.
2. Description of the Related Art
There is a demand for boards of a predetermined thickness and having a plurality of small, long and narrow perforations precisely formed therein. For example, in case of conductive vias in a wiring board for mounting electronic components thereon, an ink discharge portion of an inkjet printer, and the like, fine perforations having a diameter of not greater than an order of several tens to several hundreds of micrometers are required to be accurately and precisely formed. This is because of the common requirement that all industrial products are less expensive, lighter, and smaller. Therefore, since compactness puts an additional value to industrially produced articles, high integration in packaging circuits, parts, and so forth is now in progress. That is, more small perforations must be formed over a smaller fixed area. With these backgrounds, the perforations are getting smaller and longer. Accordingly, in the perforations, as the diameter thereof becomes smaller and smaller, and as the axial length thereof becomes longer and longer, the aspect ratio (L/D) thereof becomes higher and higher.
In general, the term “aspect ratio” means a ratio of an axial length of a perforation to a diameter thereof when the perforation has a cylindrical shape, otherwise “aspect ratio” means a ratio of the axial length of the perforation to the minimum distance between the opposing edges on the opening of the perforation. Here, the term “minimum distance between the opposing edges on the opening of the perforation” means the minimum distance S shown in FIGS. 5(a) or 5(b). That is, the expression “a perforation having a high aspect ratio” means a long narrow bore having a relatively small diameter (or a relatively small minimum distance between the opposing edges on the opening of the perforation) and a comparatively long axial length. Known methods for fabricating a perforation having such a high aspect ratio have a variety of problems.
(A) Problems in Die-Punching Processes
One of the known methods for forming a small perforation in a plate or a sheet (hereinafter referred to as a plate) is by die-punching, that is, to punch a plate having a predetermined thickness by using a punch and a die. Since this method requires a large clearance between the punch and the die, perforations with poor precision are often obtained. Also, in the case that a thick plate is punched, a large shearing force is applied thereon, especially when the perforations are to be formed densely in the thick plate and the die is required to have a large number of bores. Therefore, the die is often insufficient to bear the large shearing force and is deformed due to insufficient stiffness. In the worse cases, the die is broken due to the insufficient stiffness.
FIGS. 3(a) and 3(b) illustrate perforating a plate by a known die-punching method. As shown in FIG. 3(a), when a punch 10 punches a plate 13 placed on a die 12 while providing a clearance 16 between two lines extending along two edges 14 of the punch 10 and the die 12, a crack 15 occurs in the region of the clearance 16, thereby causing a variation in precision of the perforation in the region of the clearance 16. Consequently, the method for forming a perforation by die-punching generally causes the perforation in the punched plate 13 to have a tapered sectional shape widening toward the punching direction as shown in FIG. 3(b). Furthermore, the thicker the plate 13, the larger the clearance 16 is required to be, causing the perforation to have lower precision.
(B) Problems with Laser Processing
Another known method is to perforate a plate by laser processing. Since a laser beam is employed in this process, the perforation is formed by condensing a beam light in a lens and irradiating a target material with the condensed light beam. Therefore, in this laser beam processing, the perforation formed has a tapered shape that narrows in the traveling direction of the beam light as a result of the condensing method itself, which is a basic principal of this laser processing method. Accordingly, there is the fatal problem that as the aspect ratio of the perforation increases, the precision of the perforation decreases.
FIGS. 4(a) and 4(b) illustrate perforating a plate by known laser processing. As shown in FIG. 4(a), in a laser processing machine, a parallel beam 17 passes through a condenser 18, and is condensed at a focal point on a focal distance 20 from the condenser 18 so as to process the target material. The farther the target material is placed from the focal point, the wider a laser beam width 19 becomes, resulting in a larger diameter of the processed perforation. Accordingly, when a perforation is being formed on the output side of the plate 13 in the traveling direction of the beam, the diameter of the perforation formed on the input side of the plate 13 would become larger, while it would be varied, depending on the thickness of the plate 13. This would result in forming a perforation having a tapered shape that narrows in the traveling direction of the beam light as shown in FIG. 4(b).
In addition, thermal energy used for laser processing causes the plate, as a work piece, to be deformed and to thereby form a degeneration layer, leading to a problem of a variation in the diameter of the perforation. In this problem, as the thickness of the plate increases, the precision of the perforation becomes lower, since larger laser beams, i.e., laser beams having a larger amount of thermal energy, are required when the plate is thicker.
(C) Problems with Drilling
Further, another known perforation formation method is drilling. During the drilling process, chips produced by drilling are discharged outside through a drilled bore, causing a problem in which the chips hit the inner surface of the bore when traveling in the drilling direction of the drilled bore and accordingly widen the bore. Especially when the plate is made from a soft, plastic-deformable material, the precision of the perforation is likely to deteriorate. The higher an aspect ratio of the perforation, the longer the discharging path for the chips becomes, resulting in even lower perforation precision.
(D) Problems with Stacking Plates after being Perforated
An improved fabrication method is to perforate thin plates one by one, for example, by one of the above known methods, and then to displace and stack the perforated plates so as to achieve a multilayer board having a predetermined thickness. In this method, the bore of a single plate is more precise since the plate to be perforated at one perforating step is thin. Accordingly, it was considered that a perforation formed in the multilayer board fabricated by stacking the perforated thin plates is highly precise even when the perforation has a high aspect ratio.
However, the above method requires an additional jig for displacing the thin plates, an additional space for stacking the thin plates, and an additional step, which lowers production efficiency and increases production costs. Also, a guide pin is required for precise stacking, ending up as an extra bore in each thin plate in addition to the perforation necessary for the plates. Furthermore, a thin plate especially made from a soft material is easily deformed while being displaced and stacked, thereby causing the bore to be deformed. Accordingly, when the perforation is formed in the multilayer board having a predetermined thickness by aligning the bores perforated in the stacked thin plates, the precision of the perforation becomes lower. Therefore, a further improvement is desired, since there is a limitation on the precision of the perforation formed in the plates having a predetermined thickness.
A further elucidation will be made by taking as a specific example a circuit substrate represented by a printed wiring board and the like.
It has been eagerly desired to form a through-hole (perforation) having a more preciseness in a circuit substrate. Here, the term “a through-hole” means a bore passing through from the top to the bottom of the circuit board. For example, many of the latest wiring boards are multilayered in response to the requirement for high density integration. In general, a multilayer printed wiring board is formed by stacking circuit conductors with insulators. The conductive vias are formed in the insulators for electrically connecting the adjacent conductors, i.e., circuits on different layers, with each other by interposing the corresponding insulators between the circuits. A through-hole is also formed, passing through all the layers, so as to act to connect the conductors to achieve interlayer connection together with the vias.
The through-hole is mainly used to connect the outermost layers of the stacked conductors with each other after formation of the conductivity therein, for example, by copper plating. The through-hole is expected to prevent defects such as an electrical disconnection and an increase in resistance in order to prolong the duration of life and improve the reliability of the circuit substrate. Therefore, the through-hole is filled with and sealed by an insulating material such as an epoxy resin if required so as to prevent occurrence of cracks and the like caused by an external disturbance such as thermal expansion or stress during the fabrication or the practical use.
However, in the case of the conventional multilayer printed wiring board having a sealed through-hole therein, there still remains a problem of lowered yields due to cracks in the through-hole. As discussed below, it was presumed that trouble with the precision of the through-hole of conventional multilayer printed wiring boards was attributed to the fabrication method of the board. An exemplary multilayer printed wiring board and an exemplary known method for fabricating the board will be described below.
FIG. 10 is a sectional view of an exemplary multilayer printed wiring board. A multilayer printed wiring board 100 comprises insulator 71 of four plates stacked in four layers. The plates are composed of, for example, an epoxy resin and a glass cloth, and the circuit is a conductor 73 made from, for example, copper. The circuit, i.e., the conductors 73, which are interposed between the corresponding insulator 71 layers, are connected with each other by corresponding conductive vias 76 passing through the insulator 71. Also the multilayer printed wiring board 100 has a through-hole 74 passing through the four layered insulator 71 and the through-hole 74 is provided with conductivity by plating, for example, copper serving as the conductor 73. In addition, the through-hole 74 is sealed with filler 75 such as an epoxy resin. The through-hole 74 has a substantially cylindrical shape, a diameter ranging from about 50 to 100 μm, an axial length, i.e., a thickness of the multilayer printed wiring board 100, ranging from about 500 to 1000 μm, and an aspect ratio of about 10:1.
Such a multilayer printed wiring board can be fabricated, for example, as described below.
(1) First Example Fabrication Method
FIGS. 7(a) to 7(e) illustrate an example of a known fabrication method. Four plates for the insulator 71 are prepared as shown in FIG. 7(a). In a perforating step shown in FIG. 7(b), each plate of the insulator 71 is provided with respective via-holes 72 and holes 77 perforated therein; the holes 77 becoming the through-hole 74 after stacked. Perforation is usually performed by a known method such as a punching die, a laser, a drill and the like. Then, in a conductive-path forming step shown in FIG. 7(c), the via-hole 72 is filled with the conductor 73, and a thin film of the conductor 73 is formed on the inner surface of the hole 77. Conductive-path forming is usually performed by a known method such as screen printing or plating. In a circuit forming step shown in FIG. 7(d), a circuit having a predetermined pattern is formed with the conductor 73 formed on the insulator 71. Circuit forming is usually performed by a chemical process (photo etching) including resist-applying, exposing, developing, and etching; however, screen printing, plating, and so forth can be applied. In a stacking step shown in FIG. 7(e), the four plates of the insulators 71 having previously designed respective circuits formed in the steps illustrated in FIGS. 7(a) to 7(d) are stacked so that the multilayer printed wiring board 70 having the through-hole 74 passing through from the top to the bottom of the stacked plates is formed.
The multilayer printed wiring board can be fabricated by the following method having an order of steps different from that of the first exemplary fabrication method.
(2) Second Example Fabrication Method
FIGS. 8(a) to 8(e) illustrate another example of a known fabrication method. Four plates of the insulator 71 are prepared as shown in FIG. 8(a). In a circuit forming step shown in FIG. 8(b), only the via-holes 72 are perforated in the insulators 71, and the holes 77 to be acting as the through-holes 74 are formed after the insulator plates are stacked. Each of the via-holes 72 is filled with the conductor 73, and a circuit having a predetermined pattern is formed with the conductor 73 on each of the insulators 71. In a stacking step shown in FIG. 8(c), the four insulators 71 having previously designed circuits formed in the step illustrated in FIG. 8(b) are stacked to give a laminate 78. Subsequently, in a perforating step shown in FIG. 8(d), the through-holes 74 are perforated through each layer of the laminate 78. Finally, in a conductive-path forming step shown in FIG. 8(e), a film of the conductor 73 is formed on an inner surface of the through-hole so that a multilayer printed wiring board 80 having the through-hole 74 passing through from the top to the bottom of the laminate 78 is given.
As for the above described fabrication methods, when a laminate is provided with the through-hole 74 by forming the holes 77 in the plates and stacking the holes 77 according to the first exemplary fabrication method described (1), the precision of each of the holes 77 is high. However, the problems caused by stacking after being perforated as described above remain unsolved. That is, a displacement (i.e., a stepped portion) is formed between the stacked plates when the insulators 71 having the conductors (circuits) 73 formed thereon are stacked. When a thick stacked laminate is perforated according to the second exemplary fabrication method, however, the problems described with respect to die-punching, laser processing, drilling, and stacking after perforated remain unsolved. As a result, the precision of the through-hole 74 is limited in many cases.
Accordingly, cracking and/or pealing of the conductor is likely to occur at the step portions of the through-hole formed at stacked portions due to external disturbances such as thermal expansion and stress, which causes troubles such as an electrical disconnection or an increase in resistance, thereby leading to the shortened life time and the lowered reliability of a circuit substrate. In particular, such troubles are likely to occur in a circuit substrate densely mounted on a computer packaged, since the circuit substrate is stacked in several tens of layers.
When the through-hole of the circuit substrate is actually wider or narrower beyond a designed tolerable value, the circuit substrate is defective since the circuit substrate has a conducting portion which was originally designed to be insulative or an insulating portion which was originally designed to be conductive. This trouble is likely to occur in a printed wiring board stacked in several tens of layers as described above, since it is required to have the predetermined thickness.
Furthermore, the reduced smoothness of the inner surface of the through-hole due to the step portions at the stacked portions causes an increase in resistance and noise generated due to inductance and/or capacitance components. Such noise can be fatal to a circuit substrate used for a computer having an energy-saving circuit therein in which an extremely minute electric current flows. A circuit substrate having a precise through-hole in which the step portions at the stacked portions of the through-hole are small prevents these problems.
As described above, multilayer wiring boards, for example, having an extremely fine, long, narrow, and precise through-hole, a multilayer board having a predetermined thickness and a precise perforation with a high aspect ratio have been required, even when the multilayer board is formed from a soft material deformable due to its dimensions or its shape. However, there has been no proposal for achieving such a multilayer board.